In powder metallurgy of using a metal powder such as iron powder or steel powder as the essential material, a powder such as an alloying ingredient or graphite powder is added to and mixed with the essential material powder as the component for improving the physical properties (strength characteristic and workability characteristic) of the sintered body, then a lubricant is further added thereto and this is shaped by compression to give a green compact, and thereafter the green compact is sintered into a sintered body. In the powder metallurgy method, when the mixed powder is discharged out of a storage hopper or when the mixed powder is filled into a mold, the flowability of the mixed powder is one important characteristic factor. Specifically, when the flowability of the mixed powder is poor, then it may cause some problems in that the powder may bridge the upper area of the hopper discharge port, there by resulting in discharge failure, and that the powder may clog the hose from the hopper to a shoebox. Even when a mixed powder of poor flowability could be forcedly discharged out through a hose, it could not fill a mold, especially the thin-wall area of a mold and therefore a good shaped article could not be produced. Accordingly, the demand for a mixed powder of good flowability is strong.
It is considered that the flowability of a mixed powder may depend on the particle size and the shape of the metal powder used, the type and the amount as well as the particle size and the shape of an additive element to be added thereto for improving physical properties, but may be influenced mostly by the type and the amount of a lubricant to be added to it. The uppermost limit of the amount of the lubricant may be generally up to 0.1% by mass, and with the increase in its amount, the flowability of the mixed powder may worsen. Therefore, from the viewpoint of the flowability of the mixed powder, the amount of the lubricant to be added thereto is preferably lower. However, when the amount of the lubricant is lowered, then the lubricity of the mixed powder may extremely lower, and as a result, when the shaped article is taken out of a mold, then the friction coefficient between the shaped article and the mold surface may increase whereby the article may be galled by the mold or the mold may be damaged. Accordingly, it has been difficult to satisfy both the lubricity and the flowability in powder metallurgy.
From the viewpoint of the type and the melting point of the lubricant to be used therein, it is also difficult to satisfy both the lubricity and the flowability in powder metallurgy. Specifically, stearic acid and stearic acid amide having a low melting point generally have good lubricity, but when such a low-melting-point lubricant is used, then the powder may aggregate and its flowability may worsen. In particular, the failure is remarkable when the ambient temperature is high. On the contrary, metal soap and ethylene-bisamide having a high melting point could keep good flowability even at a high ambient temperature, but their lubricity is inferior to that of the above-mentioned low-melting-point stearic acid amide, etc.
For satisfying both the flowability and the lubricity, for example, JP-A-10-317001 is known. In this publication, the surfaces of metal powder particles are coated with an organic compound (e.g., organoalkoxysilane, organosilazane, titanate-type or fluorine-containing coupling agent) that is stable even up to a high-temperature range (about 200° C.), whereby the frictional resistance thereof is reduced and the contact charge thereof is prevented so as to improve the flowability of the particles. In addition, the publication says that the compound may also improve the lubricity of the particles. Further, the publication says that the organoalkoxysilane and others may react with the hydroxyl groups existing in the surfaces of the metal powder particles through condensation to form chemical bonds for surface modification. However, the method in this publication requires the complicated step (for pretreatment) of previously spraying the organic compound on the metal powder particles so as to coat their surfaces, additionally requiring removal of the solvent used for the coating (spraying) by drying the particles, and therefore it is unsuitable for industrial-scale production.
In JP-A-10-317001, a fatty acid monoamide (e.g., ethylene-stearic acid monoamide) or a fatty acid bisamide (e.g., ethylene-stearic acid bisamide) is additionally used as a lubricant. However, the lubricant is ineffective for improvement of flowability, as so mentioned hereinabove.